Here is what I am trying to do. I got an armature consisting of three bones. I IK constrained the tail bone to a dummy and when I move the parent bone, the armature should move in a way that the angle between the middle bone and the tail is fixed. I have tried all kinds of tricky stuff, but couldn't get it work.

Here is what I mean (sorry for the annoying logo!).

Basically, I need the angle between the black bone and yellow bone preserved. It is possible whenever the parent bone (blue) and the IK bone (yellow) rotate "more" so to speak. I mean, it's possible mechanically.

$\begingroup$can you use one single bone for the middle and the tail? From the looks of the object, it really is only one piece.$\endgroup$
– David♦Jan 25 '16 at 23:20

$\begingroup$Maybe, I am using the terminology wrong, but from my perspective a bone is linear piece of an armature. There are three bones in my situation.$\endgroup$
– Rubi ShnolJan 26 '16 at 6:42

$\begingroup$If you never want that portion of the object to bend, you can treat the whole part as one bone ... regardless of it's shape. So take out the bone you don't need and simplify the rig.$\endgroup$
– hawkenfoxJan 26 '16 at 8:46

$\begingroup$@hawkenfox Ok. I need this configuration of the bones. I bind an object to the IK bone and I need it move in the said manner.$\endgroup$
– Rubi ShnolJan 26 '16 at 8:48

$\begingroup$@ValerySaharov You only need this configuration if those 2 bones (black and yellow) will ever move/rotate/scale against each other. Do they? Do you need the joint in there? If yes I'll write a solution for you but if no just use single bone there.$\endgroup$
– Jaroslav Jerryno NovotnyJan 26 '16 at 8:54

$\begingroup$Exactly! For some reason, I've tried the same with the black bone and it didn't work. Seems to be not so intuitive as to why exactly the yellow bone's rotation needs to be locked.$\endgroup$
– Rubi ShnolJan 26 '16 at 10:01

In IK rigging terms, it's the IK object that animator work with. The
other bones "reacts" to the movement of the IK object. You will often
find that when animator work on an IK rig, they hide all the bones
that aren't IK object ... simple because they don't directly interact/transform them.

$\begingroup$It would be what I need, if arm.lower's tail were fixed as in my picture.$\endgroup$
– Rubi ShnolJan 26 '16 at 9:34

$\begingroup$I need it to do what it does on my picture when I move the blue bone horizontally, not the IK object.$\endgroup$
– Rubi ShnolJan 26 '16 at 9:41

$\begingroup$The moved IK target has different rotation origin and the constraint solution does not affect IK rotations - only Pose rotations - so that does not work at all, the constraint is redundant there.$\endgroup$
– Jaroslav Jerryno NovotnyJan 26 '16 at 10:02

$\begingroup$I agree with your remark. I just omitted another IK that should have been at the blue bone's head$\endgroup$
– Rubi ShnolJan 26 '16 at 10:03